1. Field of the Invention
The present invention relates to a liquid crystal display and a manufacturing method thereof, and more specifically to a liquid crystal display and a manufacturing method thereof, in which connection stability is improved when connecting a COG (Chip On Glass), a COF (Chip On Film), or an FPC (Flexible Printed Circuit film) to a driving circuit.
2. Description of the Related Art
In an information-oriented society these days, the role of an electronic display is getting more important. The electronic displays of all kinds are widely used in various industrial fields. As techniques of the electronic display field are continuously developed, various electronic displays having new functions are provided corresponding to diverse requirements of the information-oriented society.
Generally, the electronic display is an apparatus for visually transmitting information to a person. That is, an electric information signal output from various electronic equipments is converted into a visually recognizable optical information signal in the electronic display. Therefore, the electronic display serves as a bridge for connecting the person and the electronic equipments.
The electronic display is classified into an emissive display that displays the optical information signal by emitting lights, and a non-emissive display that displays the signal by optical modulation such as light-reflecting, dispersing and interference, etc. The emissive display is called an active display. Examples are a CRT (Cathode Ray Tube), a PDP (Plasma Display Panel), an LED (Light Emitting Diode) and an ELD (Eelectroluminescesnt Display), etc. The non-emissive display is called a passive display. The examples are an LCD (Liquid Crystal Display) and an EPID (Eelectrophoretic Image Display), etc.
The CRT has been used for image display such as in a television and a monitor, etc., over the longest period of time. The CRT has enjoyed the highest market share because of high display quality and low costs. However, it also has much disadvantage such as heavy weight, large volume and high power consumption.
Meanwhile, due to rapid development of semiconductor technologies, various kinds of electronic devices are driven at lower voltage and consuming less power, rendering the electronic equipments much slimmer and lighter. Therefore, a flat panel type display of slimmer and lighter property as well as the less driving voltage and lower power consumption property is required according to the new environment.
An LCD, among various flat panel type displays, is much slimmer and lighter than any other displays. It can be driven at a lower voltage and consume less power. It also shows high display quality similar to the CRT. Therefore, the LCD is widely used in various electronic equipments. Further, since the LCD can be facilely manufactured, its application is getting wider. The LCD is classified into a backlight LCD that displays an image using an external light source and a reflective LCD that displays the image using ambient light instead of the external light source. Methods of manufacturing the backlight LCD and the reflective LCD are disclosed in Korean Paten Laid-Open Publication Nos. 1999-18395 entitled “Method of manufacturing polycrystalline silicon thin film transistor”, 2000-66398 entitled “Method of manufacturing TFT LCD panel” and 2000-59471 entitled “Reflective type LCD and manufacturing method thereof”.
FIGS. 1A, 113, and 1C are cross-sectional views disclosing a conventional method of manufacturing the LCD.
Referring to FIG. 1A, a metallic material such as Al and Cr, etc., is deposited on a substrate 10 of an insulating material, and then patterned to form a gate electrode 15 and a gate terminal 20. Continuously, a gate insulating layer 25 is formed on the entire surface of the substrate 10, where the gate electrode and terminal 15, 20 are formed, by a PCVD (plasma chemical vapor deposition) process.
Thereafter, an in-situ doped n+ type amorphous silicon film is deposited on the gate insulating layer 25 and then patterned to form an amorphous silicon layer 30 and an ohmic contact layer 35 on the gate electrode 15.
The metallic material such as Mo, Al, Cr or W, etc., is further stacked on the gate electrode 15 and then patterned to form a source electrode 40 and a drain electrode 45. At this time, in a pad area 70 of the substrate 10, there is formed a data input terminal (not shown). Thus, in an active region 50 of the substrate 10 except the pad area 70, is formed a thin film transistor 60 including the gate electrode 15, the amorphous silicon layer 30, the ohmic contact layer 35, the source electrode 40 and the drain electrode 45.
Referring to FIG. 1B, an organic photoresist layer is stacked on the entire surface of the active region 50 and the pad region 70 of the substrate 10 to form a protective layer 75. Thus, the lower substrate 10 is completed.
Referring to FIG. 1C, in order to form a contact hole 80, 81, a mask (not shown) is positioned on an upper portion of the protective layer 75. Then, the contact hole 80, 81 is formed on the protective layer 75 by an exposing and developing process so as to partially expose the drain electrode 45 and the gate terminal 20.
Afterwards, the metallic material such as Al or Ni, having a high reflectivity, is deposited in an inner portion of the contact hole 80, 81 and on the organic insulating layer (protective layer) 75. The deposited metallic material is patterned in the form of a desired pixel to form a reflective electrode 85 and a pad 86. Then, an alignment layer is formed thereon. An upper substrate (not shown) including a color filter, a transparent electrode and the alignment layer is formed facing the lower substrate 10.
The upper substrates and the lower substrate are put together with spacers interposed therebetween. A liquid crystal layer is formed at a space between the upper substrate and the lower substrate to complete the LCD.
The completed LCD is connected to a connecting device such as a COG, a COF or an FPC, etc., so as to apply a driving signal through the pad 86 from an outside.
However, in the above-mentioned conventional method of manufacturing the LCD, since the organic insulating layer or other thick layer is used as the protective layer of the thin film transistor, a step difference is generated between a pad portion under which the metal layer is formed and a remaining portion. Therefore, there is a problem that a pressing failure occurs due to the step difference, when connecting a bump, etc., of the COG, the COF or the FPC to the pad portion.
FIG. 2A is a plan view of a conventional pad structure having the step difference by opening the contacts according to each terminal, and FIG. 2B is a cross sectional view taken along a line A-A when connecting the bump by a pressing process.
Referring to FIGS. 2A and 2B, in the conventional individual terminal opening type pad structure, a pad contact hole 102 having a little smaller surface area than that of a lower terminal 100 is formed in a protective layer 106. Then, a pad 104 having an area a little wider than the surface area of the terminal 100, is formed in order to electrically connect the terminal 100 and the pad 104.
As a result, the protective layer is thickly formed in a thickness of about 5 pm, the terminal of the pad contact hole 102 is formed about 3-4 pm high. An adhesive resin (ACF:anisotropic conductive film) 108a containing a conductive ball 108b is coated thereon. A bump 110 connected to a terminal part of a driver IC is pressed on the ACF 108a. Therefore, the pad 104 and the bump 110 are electrically connected to each other by the conductive ball 108b compressed therebetween.
As shown in FIG. 2B, however, since only a peripheral region of the pad contact hole is electrically connected by the step difference of the pad contact hole 102, and the conductive ball 108b is not fully compressed at the center of the pad 104, an electrical connection may fail. Therefore, a contact resistance generally increases, thereby lowering electrical properties.
In addition, if a misalignment between the bump and the pad occurs, the contact resistance further increases. The high contact resistance at the contact portion generates a large amount of resistance heat. As the result, the contact is cut off and thus the reliability of the device is lowered.
Therefore, in order to solve the above problem, there has been provided a terminal batch opening method. FIGS. 3A shows a plan view of a conventional flat pad structure formed by collectively opening the terminals and FIG. 3B shows a cross sectional view of the flat pad structure when connecting a bump by a pressing process.
Referring to FIGS. 3A and 3B, an opening 112 including the whole terminals is formed on the protective layer to open the plurality of terminals. After depositing a pad conductive material thereon, a photolithography process is performed to form a pad pattern every terminal. Therefore, a flat pad 104 without a contact step difference is formed on the terminal 100. In this method, all of the conductive balls 108b is fully compressed between the bump 110 and the pad 104, thereby improving the contact capability there between.
However, as shown in FIG. 3B, if the bump 110 is misaligned, the protective layer between the terminals 100 is removed due to the opening 112, and thus the conductive ball 108b is compressed at a portion in which the bump 100 is overlapped with an adjacent terminal, as shown in an “X” portion of FIG. 3B. Therefore, two terminals are electrically connected with one bump at the same time, causing contact failures.
Further, as shown in a “Y” portion of the FIG. 3B, when the opening 112 is formed where a data input terminal is formed, an under-cut portion is formed at an insulating layer of a lower portion of the terminal 100. Therefore, the terminal 100 tends to peel off, or the adhesive resin 108a is not sufficiently coated under the under-cut portion, exposing the under-cut portion to the outside. Also, moisture or contaminant infiltrates through the exposed portion and electrochemically reacts with a metal portion of the terminal to cause corrosion of the metal portion.